<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
    var str="hello world";
    console.log(str.length)
    var newStr=str.concat(123);
    console.log(newStr);
    console.log(str.charAt(0));
    console.log(str.indexOf("d"));
    console.log(str.lastIndexOf("l"));
    </script>
</body>
</html>